In order to produce driver assistance systems such as, for example, a traffic sign recognition system or a lane departure warning system, camera systems, in particular stereo cameras, are used for improved depth resolution. The term ‘stereo camera’ denotes two cameras each comprising a lens optical unit and an image sensor, which are mounted offset with respect to one another at a predetermined distance, for example on a windshield of the vehicle. The advantage of stereo cameras is that they can be used to measure distances.
In order to realize the aforementioned driver assistance functions, camera systems require a horizontal opening angle of approx. 50° and a vertical opening angle of approx. 30°. Future camera systems require a significantly higher opening angle for new functions such as cross-traffic detection or traffic light detection.
Camera systems having a high resolution and, at the same time, a large opening angle require image sensors with a high number of pixels of multiple megapixels, which are already standard for image photography. However, such image sensors are not suitable for automotive usage because, on the one hand, they are too insensitive due to the small pixel size and, on the other hand, they do not allow efficient processing of the large number of pixels for the elaborate image processing algorithms.
In this technical context the generic US patent specification U.S. Pat. No. 8,305,431 B1 describes a stereo camera for a vehicle having a first and a second camera which, however, have different and overlapping camera fields of view (FOV) in order to produce multi-functionality. One of these cameras is designed as a black and white camera and the other is designed as a color camera, wherein the black and white camera has an opening angle between 30° and 50°, preferably 40°, while the color camera has an opening angle between 15° and 25°, preferably 20°. Identical image sensors, in particular in terms of their resolution, are proposed for both cameras.
DE 10 2004 061998 A1 also describes a similar system of a stereo camera, which comprises a black and white camera having an opening angle of 40° as well and a color camera having an opening angle of 70°. The black and white camera has a gray-scale value image sensor with a high light sensitivity and a pixel size of 8 μm at a horizontal geometric resolution of 800 pixels. The color camera is equipped with a high-resolution color image sensor with a pixel size of 4 μm at a horizontal geometric resolution of 1600 pixels. However, the use of such a high-resolution color image sensor is contrary to an inexpensive realization of driver assistance functions.
A stereo camera is also known from US 2013/0120538 A1, the two cameras of which are equipped with different image sensors with respect to the number of pixels. In addition, lens systems for adjusting the camera fields of vision thereof are assigned to each camera. The one lens system results in an opening angle of 55° and the other lens system results in an opening angle of 80°.
In “Panomorph Based Panoramic Vision Sensors”, in the publication “Vision Sensors and Edge Detection”, Francisco Gallegos-Funes (ed.), ISBN: 978-953-307-098-8, InTech, Simon Thibault (2010) describes so-called “panomorph optics” having increased resolution in predefined zones of the camera field of view of the related image sensor, which constitutes a particular type of panoramic lenses. These are characterized by two parameters, namely the amount and location of the resolution within the panoramic camera field of view. The resolution constitutes a design parameter for such panomorph lenses, in order to realize a high-resolution pre-determined zone, i.e. the camera field of view is divided into different zones which have different resolutions. A panomorph lens is therefore described, the central zone of which has a resolution which is twice that of the resolution at the periphery, as is explained with reference to FIGS. 8 and 9.
FIG. 8 shows a camera viewing window having an opening angle of +/−90° about an optical axis O with a high-resolution central region Z corresponding to an opening angle of +/−30° and peripheral zones Z′ and Z″ with a lower resolution. FIG. 9 shows the graph of the resolution as a function of the opening angle, wherein the central region Z has double the resolution of the region Z″ lying outside the central region. These FIGS. 8 and 9 also show a transition region Z′ between the central region Z and the range Z″, which extends over an angular range of 10°.
The author of the aforementioned article also explains the application of an image sensor having a panomorph lens for providing the necessary image information for a driver assistance system.